In a molder, for example, a molder disclosed in U.S. Pat. No. 4,105,391, wherein a plurality of neck molds are mounted at given intervals at the undersurface of a transferring platen, the neck molds and injection molds are closed, injection cores are inserted, molten resin is injected towards the inside of the neck molds to mold a preform with a bottom around the injection cores, and thereafter the injection cores are removed and transferred together with the neck molds to the succeeding step after the preform has been opened. If the injection molding stage is expanded into the range including stopping positions of two neck molds and the injection molds for molding an inner layer and an outer layer of the preform with a bottom are disposed on the neck molds, as described in U.S. Pat. No. 4,321,029, the inner layer preform and the outer layer preform can be injection molded continuously, and the double-layered preform can be transported to the succeeding step in the state without being modified.
However, in the molder which transfers the preform while being held by the neck mold, the molten resin is poured to the inside of the neck mold to mold the inner layer preform first. Therefore, in the next step of molding the outer layer preform, a space for receiving the poured molten resin cannot be formed between the neck mold and the inner layer preform without modification, and the outer layer preform molded therein is in the range molded by the injection mold. Thus, another means has to be used to mold a double-layered preform 1, that is, a preform in which an inner layer 2 and an outer layer 3 are formed up to a neck portion, as shown in FIG. 1.
The necessity of double-layered containers such as bottles for beverages is increasing with the wide use of synthetic resin containers. Particularly, in double-layered containers with an inner layer formed of polyethylene-terephthalate which is liable to give rise to thermal deformation when a filling temperature of content exceeds 80.degree. C., if the neck portion is formed by the inner layer alone, even if the outer layer is formed of polycarbonate of excellent heat resistance, the neck position undergoes thermal deformation at the time of filling to break a seal of a lip portion, failing to serve as a container. However, if the neck portion is formed on both inner and outer layers, deformation of the neck portion can be prevented by polycarbonate of the outer layer.